Syn- and post-eruptive erosion, gully formation, and morphological evolution of a tephra ring in tropical climate erupted in 1913 in West Ambrym, Vanuatu

Abstract

Syn-and post-eruptive erosion of volcanic cones plays an important role in mass redistribution of tephra over short periods. Descriptions of the early stages of erosion of tephra from monogenetic volcanic cones are rare, particularly those with a wellconstrained timing of events. In spite of this lack of data, cone morphologies and erosion features are commonly used for long-term erosion-rate calculations and relative age determinations in volcanic fields. This paper offers new observations which suggest differing constraints on the timing of erosion of a tephra ring may be operating than those conventionally cited. In 1913 a tephra ring was formed as part of an eruption in west Ambrym Island, Vanuatu and is now exposed along a continuous 2.5 km long coastal section. The ring surrounds an oval shaped depression filled by water. It is composed of a succession of a phreatomagmatic fall and base-surge beds, interbedded with thin scoriaceous lapilli units. Toward the outer edges of the ring, base-surge beds are gradually replaced in the succession by fine ash-dominated debris flows and hyperconcentrated flow deposits. The inter-fingering of phreatomagmatic deposits with syn-volcanic reworked volcaniclastic sediments indicates that an ongoing remobilisation of freshly deposited tephra was already occurring during the eruption. Gullies cut into the un-weathered tephra are up to 4 m deep and commonly have c. 1 m of debris flow deposit fill in their bases. There is no indication of weathering, vegetation fragments or soil development between the gully bases and the basal debris flow fills. Gully walls are steep and superficial fans of collapsed sediment are common. Most gullies are heavily vegetated although some active (ephemeral) channels occur. These observations suggest that the majority of the erosion of such tephra rings in tropical climates takes place directly during eruption and possibly for only a period of days to weeks afterward. After establishment of the gully network, tephra remobilisation is concentrated only within them. Therefore the shape of the erosion-modified volcanic landform is predominantly developed shortly after the eruption ceases. This observation indicates that gully erosion morphology may not necessarily relate to age of such a landform. Different intensities of erosion during eruption (related to water supply or rainfall) are probably the major influence on gully spacing, modal depth and form. Longer-term post-eruption processes that could be indicators of relative age may include internal gully deepening (below basal debris flow fill sediments) and possibly widening and side-slope lowering due to undercutting and side-collapse.

Intra- and extra-caldera volcaniclastic facies and geomorphic characteristics of a frequently active mafic island–arc volcano, Ambrym Island, Vanuatu

“…Scales and styles of volcanism on the island appear to show a steady state of activity, similar to detailed historic observations over the last c. 100 years, and occasional reports going back to 1774 when Captain Cook visited the region (Gregory, 1917;McCall et al, 1970). Historicallydescribed flank eruptions were always coincident with or followed initial summit eruptions, suggesting a strong coupling mechanism (Purey-Cust, 1896;Frater, 1917;Gregory, 1917;Németh and Cronin, 2007). Magma rise in the island's centre and the presence of a shallow magma storage region below the central edifice is indicated by the constant low-level lava lake activity in the intra-caldera vent systems as well as the huge rates of CO 2 , SO 2 and halogen degassing from the centre (e.g.…”

A combined field and numerical approach to understanding dilute pyroclastic density current dynamics and hazard potential: Auckland Volcanic Field, New Zealand

“…The massive, poorly sorted nature of the LT1-TB1 tuff breccia suggests local (near source) transport in a highly concentrated flow, such as a remobilized debris flow (e.g., lahar). This type of deposit is common in proximal regions of maar volcanoes and often occurs as syn-eruptive debris flow filled gullies (e.g., Nemeth and Cronin, 2007;Brand et al, 2008). This lithofacies is important to note here as its proximal nature suggests that the Maungataketake eruption may have been initiated on the northwest side NOTICE: this is the author's version of a work that was accepted for publication in Journal of Volcanology and Geothermal Research.…”

Volcanology and eruptive styles of Barren Island: an active mafic stratovolcano in the Andaman Sea, NE Indian Ocean

“…Basaltic pyroclastic flows and their deposits are less common than those of more evolved composition, but are known, for example, from the large (25 km×18 km) Aso caldera, Japan (Miyabuchi et al 2006). Pyroclastic flows are typically associated with lava dome collapse, and move along a volcano's slopes, along channels, and the channel used or cut by such flows is often recognizable in cross-section (e.g., Németh and Cronin 2007).…”